Add ADR-002 - Mempool

Author: tzdybal

docs/lazy-adr/adr-002-mempool.md

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+# Mempool
+
+For now, mempool implementation from lazyledger-core/Tendermint will be used.
+
+## Pros
+* good integration with other re-used code (see ADR-001)
+* well tested
+* glue code is not required
+* it will be updated in case of ABCI++ adoption
+* mempool doesn't depend on P2P layer, so it's easy to replace it with libp2p
+
+## Alternatives
+* Implementation from scratch
+  * time consuming
+  * error prone
+* Re-using other mempool (Celo, Prysm, etc)
+  * different API
+  * potential licensing issues